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Group U1 Lithium Battery Guide: 12V 35Ah LiFePO4 for a U1 Case
Short answer: A Group U1 lithium battery like the Pro-Guide PGLM35 is a 12V LiFePO4 deep cycle battery built in the standard Group U1 case (7.64 x 5.20 x 6.81 in). The PGLM35 stores 35Ah and 448 watt-hours, weighs 9.48 lb, accepts up to a 35A charge, and is rated for more than 3,000 cycles at 80% depth of discharge, so it can replace a U1 lead-acid or AGM battery without changing your battery box.
Group U1 is one of the most common small battery sizes around, but it is also one of the most confusing. Shoppers search for "U1 battery" and find everything from lawn tractor starting batteries to wheelchair batteries. This guide explains what the Group U1 size actually means, what a U1 lithium battery like the PGLM35 can power, how it compares with U1 AGM and sealed lead-acid batteries, how to charge and install it, and how to choose between the 35Ah PGLM35 and the 46Ah PGLM46.
Key Takeaways
- Standard U1 size: the PGLM35 uses a Group U1 case, 7.64 x 5.20 x 6.81 in (194 x 132 x 173 mm), so it fits U1 battery boxes and trays.
- Light: 9.48 lb (4.3 kg), with a built-in carry handle.
- Usable power: 35Ah and 448Wh, about 17.5 hours on a 2A fish finder.
- Fast charging: 10A standard and 35A maximum charge current, so about 1 hour from empty at the maximum.
- Long life and long warranty: more than 3,000 cycles at 80% depth of discharge, with a 5-year free replacement and 10-year pro-rata warranty.
What Is a Group U1 Battery?
"Group U1" is a battery case size defined by the Battery Council International (BCI). Like Group 24 or Group 31, it describes the outside dimensions and general layout of the case, not what is inside. A U1 battery can be a flooded lead-acid starting battery, a sealed AGM deep cycle battery, or a lithium battery, as long as it fits the U1 envelope.
U1 is a small, compact size. You will typically find U1 batteries in small boats and kayaks, garden tractors and riding mowers, mobility scooters and power chairs, fish finder and electronics setups, and portable or backup power. Because so many of those applications have a U1-sized tray or battery box, a Group U1 lithium battery is one of the simplest lithium upgrades you can make: the case drops into the same space.
That said, the job matters as much as the size. The Pro-Guide PGLM35 is a deep cycle battery. Its label states plainly that it is not designed for engine starting. If your U1 slot is for cranking a mower or small engine, you need a starting battery, not the PGLM35.
Pro-Guide PGLM35 Specifications
| Model / SKU | Pro-Guide PGLM35 |
| Group size | U1 |
| Chemistry | Lithium iron phosphate (LiFePO4), deep cycle |
| Voltage | 12V (12.8V nominal on the label) |
| Capacity / energy | 35Ah / 448Wh |
| Cycle life | More than 3,000 cycles at 80% depth of discharge |
| Charge current | 10A standard, 35A maximum |
| Terminals | Internal thread (M6) |
| Protection | Built-in battery management system (BMS) |
| Bluetooth | Enabled |
| Dimensions | 7.64 x 5.20 x 6.81 in (194 x 132 x 173 mm) |
| Weight | 9.48 lb (4.3 kg) |
| Warranty | 60-month free replacement, 120-month total pro-rata |
Charging rules printed on the battery's top label are covered in the charging section below.
The PGLM35 fills a standard Group U1 footprint and weighs 9.48 lb, with a built-in handle for carrying it to the boat.
How Long Will a 12V 35Ah Lithium Battery Run?
For a quick estimate, divide 35Ah by your load in amps. A LiFePO4 battery can use nearly all of its rated capacity and holds steady voltage as it drains, so the estimate is much closer to real-world results than it is for a lead-acid U1.
| Load | Draw | Estimated runtime on the PGLM35 |
| Single fish finder | About 2A | About 17.5 hours |
| Two graphs or a graph plus accessories | About 5A | About 7 hours |
| Larger electronics load | About 10A | About 3.5 hours |
Treat these as rough estimates. Runtime depends on the real draw, temperature and how far you discharge the battery. The label also warns not to let the PGLM35 fall below 12V, so plan to recharge before you run it completely flat.
To compare with a lead-acid U1: a typical U1 AGM or sealed lead-acid battery is usually discharged only about halfway to protect its life. So the practical runtime of a lead-acid U1 is roughly half of its rated amp-hours, while the PGLM35 can use nearly all 35Ah.
Group U1 Lithium vs. U1 AGM and Sealed Lead-Acid
| Factor | Pro-Guide PGLM35 (LiFePO4) | Typical U1 AGM / SLA |
| Usable capacity | Nearly all of 35Ah | About 50% of rated capacity |
| Weight | 9.48 lb | Typically about twice as heavy or more |
| Cycle life | More than 3,000 at 80% DoD | Typically a few hundred cycles |
| Voltage under load | Steady until nearly empty | Drops as it discharges |
| Charge time | About 1 hour at the 35A maximum | Usually many hours, with a slow absorption stage |
| Warranty | 5-year free replacement, 10-year pro-rata | Usually 1 to 2 years |
The weight and cycle life are what most U1 owners notice first. A U1 lithium battery is easy to lift in and out, and it can outlast many lead-acid replacements. The steady voltage is what you notice on the water: electronics behave the same at the end of the day as at the start, instead of dimming, resetting or throwing low-voltage alarms.
What Is the PGLM35 Best For?
- Fish finders and boat electronics: a dedicated U1 battery for your graphs keeps sonar clean and isolated from trolling motor and engine loads. See Should You Run a Separate Battery for Your Boat Electronics?
- Kayaks and small boats: where an existing U1 box or tray is already in place.
- Light RV, camping and solar storage: 448Wh for lights, fans, pumps and device charging.
- Replacing a U1 deep cycle AGM or SLA: anywhere a U1 deep cycle battery provides steady, low-to-moderate power.
Not a fit: engine starting. The PGLM35's label says it is a deep cycle battery not designed for engine start.
The PGLM35's top label lists its charging and wiring cautions, including the note that it is a deep cycle battery, not an engine starting battery.
How to Charge a Group U1 LiFePO4 Battery
Charge time is roughly the capacity divided by the charger output:
- 10A standard charge: about 3.5 hours from empty, with a charger like the Pro-Guide 12V 10A 1-Bank Battery Charger, which has a 12V lithium profile.
- 30A charger: about 1.2 hours from empty with the Pro-Guide 12V 30A Lithium Battery Charger.
- 35A maximum: about 1 hour from empty.
The label on top of the PGLM35 adds these rules:
- Use a smart lithium charger and keep the charging voltage between 14V and 15V. Below 14V the battery cannot be fully charged; above 15V it will be overcharged.
- Do not use a charger with an automatic desulfation mode. That mode is for lead-acid batteries.
- Do not discharge below 12V, as this may damage the battery.
- Let it rest 1 to 2 hours after charging before checking the voltage. The label lists about 13.8V at full charge.
As a general LiFePO4 rule, charge in moderate temperatures and avoid charging below freezing.
Installing a U1 Lithium Battery
Confirm the fit
Group U1 is a standard size, but boxes and trays vary. Measure the length, width and height of your space and the clearance above the terminals before you install. The NOCO Group U1 Snap-Top Battery Box is sized for U1 batteries and keeps the terminals covered.
Use M6 terminal bolts
The PGLM35 has internal-thread M6 terminals. Use the correct M6 bolts and ring terminals sized for your cable, and tighten them firmly without cross-threading.
Fuse the positive cable
Install a fuse or breaker close to the battery on the positive cable, sized to your wire and equipment. It is cheap insurance for your wiring and your boat.
Stay within the label limits for multiple batteries
The label cautions against connecting more than 4 batteries in series or parallel. Use identical batteries at the same state of charge if you combine them.
Check charge from your phone
Bluetooth is listed as enabled on the PGLM35, and the product page links to the Pro-Guide Power iOS app for checking the battery.
Store it charged
Before the off-season, charge the battery and store it somewhere dry and moderate. Our marine battery maintenance guide has more storage tips.
PGLM35 vs. PGLM46: Which U1 Lithium Battery Should You Choose?
Pro-Guide makes two lithium batteries in the same Group U1 case. Here is how they differ.
| Spec | PGLM35 | PGLM46 |
| Capacity / energy | 35Ah / 448Wh | 46Ah / 588Wh |
| 2A fish finder runtime | About 17.5 hours | About 23 hours |
| Charge current (standard / max) | 10A / 35A | 10A / 46A |
| Weight | 9.48 lb | 10.14 lb |
| Case | Same Group U1 case, 7.64 x 5.20 x 6.81 in | |
| Warranty | 5-year free replacement, 10-year pro-rata | |
Choose the PGLM35 if a single day of electronics or light loads is your typical use and you want the lighter battery. Choose the PGLM46 if you run more devices, fish multi-day trips between charges, or simply want the most capacity a U1 case can give you for about two-thirds of a pound more.
Frequently Asked Questions
What size is a Group U1 battery?
Group U1 is a BCI case size. The Pro-Guide PGLM35 Group U1 lithium battery measures 7.64 x 5.20 x 6.81 in (194 x 132 x 173 mm). Always check height and terminal clearance in your space.
Will the PGLM35 fit in a Group U1 battery box?
Yes. The PGLM35 uses a Group U1 case and is designed to fit U1 battery boxes and trays. Confirm the height and terminal clearance before installing.
Can a U1 lithium battery start a lawn mower or engine?
Not the PGLM35. Its label states it is a deep cycle battery and not designed for engine starting. Use a starting battery for cranking.
How long will the PGLM35 run a fish finder?
Divide 35Ah by the draw: about 17.5 hours on a 2A fish finder and about 3.5 hours on a 10A electronics load. Actual runtime depends on the load and temperature.
How long does the PGLM35 take to charge?
About 3.5 hours from empty at the 10A standard charge current, or about 1 hour at the 35A maximum. Use a charger with a LiFePO4 profile.
How many cycles does a Group U1 lithium battery last?
It varies by model. The Pro-Guide PGLM35 is rated for more than 3,000 cycles at 80% depth of discharge.
What warranty comes with the PGLM35?
A 60-month (5-year) free replacement warranty and 120 months (10 years) of total pro-rata coverage.
Upgrade Your U1 Battery to Lithium
If your boat, kayak or power setup has a Group U1 slot and you are tired of heavy lead-acid batteries that sag and wear out, the Pro-Guide PGLM35 drops into the same space with 448Wh of LiFePO4 power, 9.48 lb of weight, fast charging and a 10-year warranty.
Not sure which U1 battery fits your setup? Call our team at 1-800-827-3232 or browse all deep cycle lithium batteries and marine accessories.
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